Triterpenoid Saponins From the Stem Bark of Pentaclethra eetveldeana

Abstract

Two previously undescribed triterpenoid saponins together with 4 known ones were isolated from the stem bark of Pentaclethra eetveldeana De Wild. & Th. Dur. Their structures were elucidated by spectroscopic methods including 1D and 2D NMR experiments in combination with mass spectrometry as 3-O-β-D-glucopyranosyl-(1→2)-[β-D-glucopyranosyl-(1→3)]-β-D-glucopyranosyl-(1→4)-β-D-glucopyranosyl-(1→3)-α-L-rhamnopyranosyl-(1→2)-[β-D-glucopyranosyl-(1→4)]-α-L-arabinopyranosyloleanolic acid and 3-O-β-D-glucopyranosyl-(1→2)-[β-D-glucopyranosyl-(1→3)]-β-D-glucopyranosyl-(1→4)-β-D-glucopyranosyl-(1→3)-α-L-rhamnopyranosyl-(1→2)-[β-D-glucopyranosyl-(1→4)]-α-L-arabinopyranosylhederagenin.

Keywords

Fabaceae triterpene saponins 2D NMR MS

The genus Pentaclethra (Fabaceae) is represented by only 3 species (Pentaclethra eetveldeana, P. macrophylla, and P. macroloba).¹ P. macroloba is known to the local people in Brazil as antidote against snakebites¹ from which triterpenoid saponins have been isolated and characterized.^1,2 P. eetveldeana is a tree of 15 to 30 m of tropical African forest. It is used in Zairian traditional medicine for the treatment of hemorrhoids and various diseases such as malaria and epilepsy.³ Previous phytochemical studies on P. eetveldeana showed the presence of saponins in the seeds⁴ with oleanolic acid or hederagenin as aglycone and glucose, rhamnose, and arabinose as sugar units. In a continuation of our studies on natural saponins from the plants of the Fabaceae family,^5,6 we decided to examine the saponins from the stem bark of P. eetveldeana De Wild. & Th. Dur. In this paper, we report the isolation and structure elucidation of 2 previously undescribed triterpene saponins together with 4 known ones. Their structures were elucidated by spectroscopic methods including 600 MHz 1D and 2D experiments (¹H, ¹³C, HSQC, HMBC, COSY, TOCSY, ROESY) in combination with HR-ESIMS and by comparison of their physical and spectral data with literature values.

The saponin fraction obtained from the 80% aqueous methanolic extract of the stem bark of P. eetveldeana was fractionated by vacuum liquid chromatography (VLC) on reverse-phase RP-18 silica gel and medium-pressure liquid chromatography (MPLC) yielding 2 previously undescribed saponins 1 and 2 (Figure 1) and 4 known ones. The known molecules were identified by comparison of their spectral data with literature values as 3-O-α-L-rhamnopyranosyl-(1→2)-[β-D-glucopyranosyl-(1→4)]-α-L-arabinopyranosyloleanolic acid⁷ and 3-O-α-L-rhamnopyranosyl-(1→2)-[β-D-glucopyranosyl-(1→4)]-α-L-arabinopyranosylhederagenin⁸ widely distributed, 3-O-β-D-glucopyranosyl-(1→3)-α-L-rhamnopyranosyl-(1→2)-[β-D-glucopyranosyl-(1→4)]-α-L-arabinopyranosyloleanolic acid,² and 3-O-β-D-glucopyranosyl-(1→3)-α-L-rhamnopyranosyl-(1→2)-[β-D-glucopyranosyl-(1→4)]-α-L-arabinopyranosylhederagenin² isolated from P. macroloba.

Figure 1

Structure of compounds 1 and 2.

Compounds 1 and 2 were isolated as white amorphous powders. The monosaccharides were identified by extensive 2D NMR and GC analyses⁹ (see the section “Experimental”) as β-D-glucopyranosyl (Glc), α-L-arabinopyranosyl (Ara), and α-L-rhamnopyranosyl (Rha) for 1 and 2. The ³ J _{H-1, H-2} coupling constants (7.6-8.2 Hz) in the ¹H NMR spectrum for the glucose in its pyranose form indicated its β anomeric orientation and the large ¹ J _H-1 _{, C-1} value of rhamnose (168 Hz) confirmed that the anomeric proton was equatorial in its α-pyranoid form. The doublet observed for the anomeric proton of arabinose in its pyranose form (³ J _{H-1, H-2} 6.4, 7.7 Hz) is consistent with the ⁴C₁ conformation of Ara with an α-orientation of the anomeric center.

Compound 1 exhibited in the HR-ESIMS a quasimolecular ion peak at m/z 1567.7140 [M+Na]⁺ (calcd. 1567.7144) compatible with the molecular formula C₇₁H₁₁₆O₃₆Na. Compound 1 showed in the ESIMS spectrum (positive-ion mode, Figure S7) an ion peak at m/z 1567 [M+Na]⁺ indicating a molecular weight of 1544. The ¹H and ¹³C NMR spectra due to the aglycone moiety displayed resonances characteristic of a triterpene with 7 angular methyl groups showing correlations in the HSQC spectrum at δ _H/δ _C 1.04 (s, H₃-23)/27.1, 0.85 (s, H₃-24)/15.6, 0.94 (s, H₃-25)/14.5, 0.82 (s, H₃-26)/16.4, 1.14 (s, H₃-27)/24.9, 0.89 (s, H₃-29)/32.2, and 0.93 (s, H₃-30)/22.6. Other characteristic signals were observed such as 1 olefinic proton at δ _H 5.22 (br t, H-12) and 1 oxygen-bearing methine proton signal at δ _H 3.11 (dd, J = 4.1, 11.1 Hz, H-3), showing correlations in the HSQC spectrum with δ _C 121.9 (C-12) and 89.1 (C-3), respectively. Extensive 2D NMR analysis confirmed the structure of the aglycone to be oleanolic acid (3β-hydroxyolean-12-en-28-oic acid) (Table 1), whose NMR data were in good agreement with literature values.¹⁰

Table 1

¹³C NMR and ¹H NMR Spectroscopic Data of the Aglycone Moieties for Compounds 1 and 2, in CD₃OD (δ in ppm)^a.

Position	1		2
Position	δ _C	δ _H (J in Hz)	δ _C	δ _H (J in Hz)
1	38.5	0.97 m, 1.61	38.2	0.96, 1.60
2	25.8	1.70, 1.82 m	25.0	1.74, 1.85
3	89.1	3.11 dd (4.1, 11.1)	81.0	3.62
4	38.9	-	42.5	-
5	55.7	0.78 br d (11.1)	46.7	1.26
6	17.9	1.40, 1.56	17.4	1.36, 1.49
7	32.6	1.30, 1.49	32.2	1.27, 1.61
8	39.1	-	39.1	-
9	47.6	1.58	47.8	1.63
10	36.5	-	36.2	-
11	23.1	1.88, 1.88	23.1	1.88, 1.88
12	121.9	5.22 br t	121.7	5.22 br t
13	144.0	-	144.0	-
14	41.5	-	41.6	-
15	27.4	1.08, 1.78	27.6	1.04 m, 1.81 m
16	22.7	1.59, 1.95 td (4.1, 13.4)	22.8	1.58, 1.96 td (4.1, 13.4)
17	46.0	-	nd	-
18	41.4	2.85 dd (3.8, 14.0)	41.6	2.86 dd (3.8, 14.0)
19	45.9	1.11 dd (2.2, 11.4), 1.68	46.2	1.10 dd (2.2, 11.4), 1.67 m
20	30.2	-	30.1	-
21	33.6	1.18, 1.38	34.0	1.17, 1.36
22	32.6	1.52, 1.73	32.6	1.52, 1.73
23	27.1	1.04 s	63.0	3.33, 3.61
24	15.6	0.85 s	12.2	0.70 s
25	14.5	0.94 s	14.9	0.96 s
26	16.4	0.82 s	16.5	0.83 s
27	24.9	1.14 s	25.0	1.15 s
28	nd	-	182.0	-
29	32.2	0.89 s	32.3	0.88 s
30	22.6	0.93 s	22.7	0.93 s

nd: not determined.

Overlapped proton NMR signals are reported without designated multiplicity.

The analysis of the ¹H NMR spectrum of the sugar part of 1 (Figure S1) showed the presence of 7 anomeric proton signals showing correlations in the HSQC spectrum at δ _H/δ _C 4.41 (d, J = 6.4 Hz)/104.4, 5.09 (d, J = 1.1 Hz)/100.7, 4.51 (d, J = 7.6 Hz)/104.2, 4.50 (d, J = 7.6 Hz)/101.7, 4.85 (d, J = 8.2 Hz)/102.1, 4.66 (d, J = 8.2 Hz)/102.8, and 4.45 (d, J = 7.6 Hz)/104.6 (Table 2). Extensive 2D NMR (Figures S2 and S3) and GC analyses (see the section “Experimental”) allowed the characterization of 1 Ara, 5 Glc (Glc1-Glc5), and 1 Rha units. Extensive 2D NMR analysis allowed the identification of the partial sequence of 1 as Glc1-(1-3)-Rha-(1-2)-[Glc5-(1-4)]-Ara-(1-3)-oleanolic acid, which was previously characterized in the genus Pentaclethra.^1,2 The remaining 3 sugar units were identified as 3 Glc (Glc2, Glc3, and Glc4). Cross peaks were observed in the COSY spectrum at δ _H/δ _H 4.50 (Glc2 H-1)/3.68 (Glc2 H-2) and in the TOCSY spectrum at δ _H/δ _H 4.50 (Glc2 H-1)/3.74 (Glc2 H-3). The protons H-2 and H-3 give correlations with their carbon signals at δ _C 79.0 (Glc2 C-2) and 86.2 (Glc2 C-3), respectively, in the HSQC spectrum, suggesting a glycosylation at these positions. The HMBC correlations (Figures 2 and S2) at δ _H/δ _C 4.85 (Glc3 H-1)/79.0 (Glc2 C-2) and 4.66 (Glc4 H-1)/86.2 (Glc2 C-3) allowed to identify the final branched trisaccharide moiety as Glc3-(1→2)-[Glc4-(1→3)]-Glc2- which was confirmed by observation of the ROESY correlations at δ _H/δ _H 4.85 (Glc3 H-1)/3.68 (Glc2 H-2), 4.66 (Glc4 H-1)/3.74 (Glc2 H-3). Finally, the HMBC correlation at δ _H/δ _C 4.50 (Glc2 H-1)/80.3 (Glc1 C-4) allowed to link the trisaccharide moiety at position 4 of the Glc1, which was corroborated by the ROESY correlation at δ _H/δ _H 4.50 (Glc2 H-1)/3.46 (Glc1 H-4). Thus, the structure of 1 was elucidated as 3-O-β-D-glucopyranosyl-(1→2)-[β-D-glucopyranosyl-(1→3)]-β-D-glucopyranosyl-(1→4)-β-D-glucopyranosyl-(1→3)-α-L-rhamnopyranosyl-(1→2)-[β-D-glucopyranosyl-(1→4)]-α-L-arabinopyranosyloleanolic acid.

Table 2

¹³C NMR and ¹H NMR Spectroscopic Data of the Sugar Moieties for Compounds 1 and 2, in CD₃OD (δ in ppm)^a.

Position	1		2
Position	δ _C	δ _H (J in Hz)	δ _C	δ _H (J in Hz)
3-O-
Ara-1	104.4	4.41 d (6.4)	104.0	4.44 d (7.7)
2	76.5	3.64	76.3	3.58
3	72.0	3.73	72.5	3.68
4	78.3	3.89	78.9	3.89
5	63.9	3.53, 4.16 dd (4.1, 12.8)	64.3	3.52, 4.19 dd (3.5, 12.9)
Rha-1	100.7	5.09 d (1.1)	100.7	5.12 d (1.1)
2	69.7	4.25 br s	70.1	4.27 br s
3	81.5	3.86	81.5	3.89
4	71.0	3.56	71.0	3.56
5	68.7	3.88	68.8	3.91
6	16.7	1.22 d (5.8)	16.7	1.25 d (5.8)
Glc1-1	104.2	4.51 d (7.6)	104.2	4.52 d (8.2)
2	73.6	3.36	73.6	3.36
3	74.6	3.56	74.8	3.56
4	80.3	3.46	80.6	3.43
5	75.2	3.47	75.2	3.48 m
6	61.1	3.65, 3.86	61.1	3.65, 3.87
Glc2-1	101.7	4.50 d (7.6)	101.7	4.50 d (7.6)
2	79.0	3.68	78.9	3.68
3	86.2	3.74	86.2	3.74
4	68.7	3.40	68.7	3.40
5	76.8	3.36	76.8	3.36
6	61.2	3.66, 3.87	61.5	3.68, 3.85
Glc3-1	102.1	4.85 d (8.2)	102.7	4.85 d (7.7)
2	74.6	3.23 t (8.7)	73.9	3.23
3	76.8	3.36	76.5	3.36
4	69.9	3.30	70.1	3.33
5	76.5	3.28	76.6	3.28
6	61.1	3.67, 3.88	61.1	3.65, 3.87
Glc4-1	102.8	4.66 d (8.2)	102.8	4.66 d (7.6)
2	73.8	3.26	73.9	3.26
3	76.7	3.36	76.5	3.36
4	70.1	3.35	70.1	3.35
5	76.6	3.30	76.6	3.28
6	61.2	3.66, 3.86	61.1	3.65, 3.87
Glc5-1	104.6	4.45 d (7.6)	104.8	4.46 d (7.6)
2	74.2	3.28	73.9	3.28
3	76.2	3.38	76.1	3.40
4	69.9	3.30	70.1	3.30
5	76.5	3.30	76.7	3.29
6	61.1	3.66, 3.87	61.1	3.65, 3.87

Overlapped proton NMR signals are reported without designated multiplicity.

Figure 2

HMBC and COSY correlations for compounds 1 and 2.

Compound 2 exhibited in the HR-ESIMS a quasimolecular ion peak at m/z 1583.7098 [M+Na]⁺ (calcd. 1583.7093) compatible with the molecular formula C₇₁H₁₁₆O₃₇Na. Compound 2 showed in the ESIMS spectrum (positive-ion mode, Figures S7) a pseudomolecular ion peak at m/z 1583 [M+Na]⁺ indicating a molecular weight of 1560. Extensive 1D and 2D NMR analysis (Tables 1 and 2, Figures S4-S6) revealed that compound 2 differed from compound 1 only by the aglycone moiety at C-23. The methyl group in 1 (δ _H/δ _C 1.04/27.1) was replaced by a primary alcoholic function CH₂OH in 2 (δ _H/δ _C 3.33, 3.61/63.0) allowing to characterize hederagenin (3β,23-dihydroxy-olean-12-en-28-oic acid) as aglycone, whose NMR data were in good concordance with literature values.¹¹ Thus, the structure of 2 was elucidated as 3-O-β-D-glucopyranosyl-(1→2)-[β-D-glucopyranosyl-(1→3)]-β-D-glucopyranosyl-(1→4)-β-D-glucopyranosyl-(1→3)-α-L-rhamnopyranosyl-(1→2)-[β-D-glucopyranosyl-(1→4)]-α-L-arabinopyranosylhederagenin.

From the crude ethanolic extract of the stem bark of P eetveldeana, we isolated 2 previously undescribed monodesmosidic triterpenoid saponins and 4 known ones by successive MPLC. As it was reported, structural analogs were isolated from P. macroloba.^1,2 Some of them shared the same 6 sugar sequence Glc-(1→2)-Glc-(1→4)-Glc1-(1→3)-Rha-(1→2)-[Glc-(1→4)]-Ara- linked at C-3 of oleanolic acid and hederagenin as in 1 and 2, which might be considered as a potential chemotaxonomic marker of the genus Pentaclethra in the Fabaceae family.

Experimental

General Procedures

Optical rotation values were recorded on AA-10R automatic polarimeter. NMR spectra were performed using a Varian INOVA 600 at the operating frequency of 600 MHz. For details, see the section Experimental part of Ref. 11.¹¹ HR-ESIMS (positive-ion mode) and ESIMS (positive-ion mode) were carried out on a Bruker micrOTOF mass spectrometer. GC analysis was carried out on a thermoquest gas chromatograph using a DB-1701 cap. column (30 m × 0.25 mm, i.d.) (J and W Scientific); detection by FID; detector temperature 250°C, injection temperature 230°C; initial temperature was maintained at 80°C for 5 minutes and then raised to 270°C at 15°C/min; carrier gas He. TLC and HPTLC are performed using silica gel plates (Merck) (CHCl₃-MeOH-H₂O, 70/30/5 and 60/32/7). The spray reagent for saponins was vanillin reagent (2% mixture of conc. H₂SO₄ soln. and 1% vanillin in EtOH). Isolations were carried out using an MPLC system (Alltech pump, Büchi column [460 × 15 mm], Büchi precolumn [110 × 15 mm], silica gel 60 [Merck, 15-40 μm]).

Plant Material

The stem bark of P. eetveldeana De Wild. & Th. Dur. (Fabaceae) was collected on the campus of the University of Kinshasa (Democratic Republic of Congo) in a small forest. It was identified by comparison with the reference Herbal P. Compere 698 kept in the Herbarium of INERA/University of Kinshasa.

Extraction and Isolation

A total of 221 g of dried and pulverized stem bark was macerated in 350 mL of 80% aqueous MeOH during 1 week and then refluxed for 1h 30. The extract was then filtered and washed with 300 mL of 80% aq. MeOH and left above a water bath to evaporate at 50°C in a ventilated atmosphere. The hydro-alcoholic extract was dissolved in MeOH (150 mL) and saponin was precipitated in Et₂O (450 mL), filtered, and dried. This procedure was repeated again, yielding 11 g of a whitish crude saponin mixture (CSM). An aliquot of CSM (1.5 g) was fractionated by VLC on normal phase silica gel (40-60 μm) using as eluent CHCl₃-MeOH-H₂O (60/32/7) to give 7 fractions Fr.1 to Fr.7 (150 mL ×7). Fr.2 (190 mg) was fractionated by MPLC over silica gel eluted with a CHCl₃-MeOH-H₂O gradient (70/30/5, 60/32/7), 2.5 mL/min, to give 12 fractions Fr.2.1 to Fr.2.12. Fr.2.11 (11 mg) was purified by VLC over RP-18 silica gel (75-200 μm) using as eluent H₂O/EtOH (linear gradient 100/0-0/100) to give compound 1 (4 mg) in the ethanolic fraction. Fr.4 (115 mg) was fractionated by VLC over RP-18 silica gel (75-200 μm) using the same gradient as above to give 2 fractions Fr.4.1 and Fr.4.2. Fr.4.2 (32 mg) was purified by MPLC over silica gel using as eluent CHCl₃-MeOH-H₂O (60/32/7), 2.5 mL/min, to give 2 (9.3 mg).

Acid Hydrolysis and GC Analysis

Each compound (3 mg) was hydrolyzed with 2 N aq. CF₃COOH (5 mL) for 3 hours at 95°C. After extraction with CH₂Cl₂ (3 × 5 mL), the aq. layer was repeatedly evaporated to dryness with MeOH until neutral and then analyzed by TLC over silica gel (CHCl₃-MeOH-H₂O 8/5/1) by comparison with authentic samples. Furthermore, the residue of sugars was dissolved in anhydrous pyridine (100 µL), and L-cysteine methyl ester hydrochloride (0.06 mol/L) was added. The mixture was stirred at 60°C for 1 hour, then 150 µL of HMDS-TMCS (hexamethyl-disilazane-trimethylchlorosilane 3:1) was added, and the mixture was stirred at 60°C for another 30 minutes. The precipitate was centrifuged off, and the supernatant was concentrated under N₂ stream. The residue was partitioned between n-hexane and H₂O (0.1 mL each), and the hexane layer (1 µL) was analyzed by GC.⁹ The absolute configurations were determined by comparing the retention times with thiazolidine derivatives prepared in a similar way from standard sugars (Sigma-Aldrich): l-rhamnose, d-glucose, and l-arabinose for 1 and 2 were characterized by co-injection of the silylated derivatives with standard silylated samples having t _R 13.1 minutes (l-rhamnose), 18.6 minutes (d-glucose), and 11.9 minutes (l-arabinose).

3-O-β-D-Glucopyranosyl-(1→2)-[β-D-Glucopyranosyl-(1→3)]-β-D-Glucopyranosyl-(1→4)-β-D-Glucopyranosyl-(1→3)-α-L-Rhamnopyranosyl-(1→2)-[β-D-Glucopyranosyl-(1→4)]-α-L-Arabinopyranosyloleanolic Acid (1)

White, amorphous powder.

$[α]_{D}^{25}$ : +2.1 (c 0.3, MeOH).

¹H NMR and ¹³C NMR (CD₃OD, 600 and 150 MHz): Tables 1 and 2.

HR-ESIMS (positive-ion mode) m/z 1567.7140 [M+Na]⁺ (calcd. 1567.7144 for C₇₁H₁₁₆O₃₆Na); ESIMS (positive-ion mode) m/z 1567 [M+Na]⁺.

3-O-β-D-Glucopyranosyl-(1→2)-[β-D-Glucopyranosyl-(1→3)]-β-D-Glucopyranosyl-(1→4)-β-D-Glucopyranosyl-(1→3)-α-L-Rhamnopyranosyl-(1→2)-[β-D-Glucopyranosyl-(1→4)]-α-L-Arabinopyranosylhederagenin (2)

White, amorphous powder.

$[α]_{D}^{25}$ : +1.2 (c 0.1, MeOH).

¹H NMR and ¹³C NMR (CD₃OD, 600 and 150 MHz): Tables 1 and 2.

HR-ESIMS (positive-ion mode) m/z 1583.7098 [M+Na]⁺ (calcd. 1583.7093 for C₇₁H₁₁₆O₃₇Na); ESIMS (positive-ion mode) m/z 1583 [M+Na]⁺.

Supplemental Material

Supporting information - Supplemental material for Triterpenoid Saponins From the Stem Bark of Pentaclethra eetveldeana

Supplemental material, Supporting information for Triterpenoid Saponins From the Stem Bark of Pentaclethra eetveldeana by David Pertuit, Mpuza Kapundu, Anne-Claire Mitaine-Offer, Tomofumi Miyamoto, Chiaki Tanaka, Clément Delaude, and Marie-Aleth Lacaille-Dubois in Natural Product Communications

Footnotes

Declaration of Conflicting Interests

The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The author(s) received no financial support for the research, authorship, and/or publication of this article.

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